It is known to form water-vapor permeable, air barrier membranes on building construction surfaces using a liquid coating composition. Such a composition is available from Henry Company, California, under the trade name AIR-BLOC 31. This composition can be spray-applied and cures to form a membrane that blocks air and air leakage and purportedly achieves a water vapor permeance of 12.3 perms (or 704 ng/Pa.m2.s) under ASTM E-96 (Henry Technical Data Sheet dated Jul. 15, 2002).
The membrane formed by the composition is believed to have a microporous structure. Microporosity is achieved by loading fillers or hard particulate materials to a level exceeding the critical “pigment volume concentration” (“PVC”). Above this critical point, the amount of filler disrupts the continuity of the polymer matrix, such that conduits are formed through which water vapor can permeate.
The Henry composition is believed to comprise approximately 15 parts calcium carbonate (a typical filler), 35 parts hydrocarbon wax (considered here to act as a filler because it does not form a film), and 50 parts vinyl acetate-acrylate copolymer, such that the critical pigment volume concentration (PVC) is exceeded.
The PVC value is determined by multiplying volume of fillers (including non-film-forming material) by 100 and dividing this product by total volume of solids. The PVC is calculated as follows: PVC=100×((wt % wax/wax density)+(wt % filler/filler density))/((wt % wax/wax density)+(wt % filler/filler density)+(wt % polymer/polymer density))=(35+(15/2.6))×100/(35+(15/2.6)+50)=45%. This figure is believed to exceed the critical PVC. Although vapor-permeable and liquid-water-impermeable, the membrane resulting from the composition is believed by the present inventor to have low elongation and poor crack-bridging properties. Furthermore, the membrane also absorbs a large amount of water.
In view of the foregoing disadvantages, the present inventor believes that novel liquid-applied vapor barrier compositions and methods are needed.